Electron trapping in electron-beam irradiated SiO2

Abstract

In addition to the well-known positive space charge, electron irradiation of MOS capacitors with 25-keV electrons is shown to introduce additional uncharged electron traps into the oxide layer. These traps persist after most of the positively charged defects have been removed by the usual low-temperature (? °C) anneals. Their presence after this anneal is determined by injecting hot electrons into the oxide where they are captured by existing defects. The effective trap densities increase with increasing electron fluence and are reduced by forming-gas anneals at temperatures in excess of 500 °C. Observed electron-capture cross sections are between 10−15 and 10−18 cm2. The residual radiation damage in oxides exposed to 10−4 C cm−2 of 25-keV electrons and subsequently annealed at 400 °C results in an additional neutral density of 5×1011 traps cm−2 with cross sections distributed over the above range. Electron-trapping cross sections and effective trap densities associated with this damage are found to be identical at 77 and 295 K. The traps are possibly associated with dipolar defects formed when valence electrons localize around an ion after the bonds are broken.